fixed bug #2399
added check expression for empty string
Merge branch 'master' into haoxuan2
Bug Fix 2381
Simplify code for typechecker.  Moved dom.byId() into
checkInitialValue.
Bug Fix 2381
Code review for inputting units bug.
Added a comment with explanation.
Adding message from PM to fix bug number 2396.
fixed sticky button issue
fixed check equation box with unknown variable in student mode issue
Merge branch 'bvds'
Code review for "sticky" clicks and equations with
unknown variables.  Combined "toPM" and "cancelUpdate" to
mean that the student entry is neither sent to the PM or
saved in the model.
Merge branch 'haoxuan2'
Merge branch 'master' into bstrong
Code review for fix targe node strategy bug #2396.
Removed redundant hint line (this is why there were two
copies of the hints).
Turn on/off sharing in AUTHOR mode

The checkbox is not reflecting the database value, but the change is
sending to database. Will fix this problem later.
css changes missed